In tuning systems employed in radio and television receivers, a received RF signal with an information bearing carrier is heterodyned with a local oscillator signal to produce an IF signal with an information bearing carrier corresponding to that of the IF signal. An automatic fine tuning (AFT) signal, having a polarity and magnitude with respect to a reference level representing the polarity and the magnitude of a deviation of the frequency of the IF signal with respect to a nominal value corresponding to correct tuning, is usually used to control the frequency of the local oscillator signal to achieve fine tuning so as to bring the frequency of the IF carrier as close as possible to the nominal value. Typically, the AFT signal has an S-shaped characteristic with positive-going and negative-going humps on opposite sides of a transition region containing the nominal frequency of the IF carrier. Tuning is considered at least approximately correct if the frequency of the IF carrier is within the transition region.
Many tuning systems are known which change the frequency of a local oscillator signal in a step-wise search until an amplitude comparator arrangement determines that one or more threshold levels of an AFT signal are achieved or traversed. For example, it is known to terminate a search when a threshold level corresponding to one of the two "humps" of the AFT characteristic is traversed in a direction going toward the transition region between the humps.